System for detecting short codes

ABSTRACT

A system employing CDMA techniques for receiving a transmitted signal wherein the transmitted signal includes a plurality of short codes, each of which is transmitted repetitively over a fixed period of time and where the received signal has CW interference in addition to the transmitted signal. The system detects the presence of the short code in a plurality of time phases of the received signal by calculating a likelihood ratio for each phase. The likelihood ratio takes into account the current short code.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/415,321, filed Oct. 8, 1999 which application is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the field of code division multipleaccess (CDMA) communication systems. More particularly, the presentinvention relates to a system for accurately detecting short codes in acommunication environment which includes continuous wave interference.

[0004] 2. Description of Prior Art

[0005] With the dramatic increase in the use of wirelesstelecommunication systems in the past decade, the limited portion of theRF spectrum available for use by such systems has become a criticalresource. Wireless communications systems employing CDMA techniquesprovide an efficient use of the available spectrum by accommodating moreusers than time division multiple access (TDMA) and frequency divisionmultiple access (FDMA) systems.

[0006] In a CDMA system, the same portion of the frequency spectrum isused for communication by all subscriber units. Typically, for eachgeographical area, a single base station serves a plurality ofsubscriber units. The baseband data signal of each subscriber unit ismultiplied by a pseudo-random code sequence, called the spreading code,which has a much higher transmission rate than the data. Thus, thesubscriber signal is spread over the entire available bandwidth.Individual subscriber unit communications are discriminated by assigninga unique spreading code to each communication link. At times it is alsouseful in a CDMA system to transmit codes which are of shorter lengththan the usual spreading code.

[0007] It is known in the art of CDMA communication systems to use asequential probability ratio test (SPRT) detection method to detect thetransmission of a short code. However, in the presence of continuouswave (CW) interference, the use of known SPRT detection methods canresult in a large number of false short code detections. These falsedetections degrade system performance by delaying the detection of validshort codes.

[0008] A background noise estimation is required for the SPRT detectionmethod. The background noise estimation is typically performed byapplying a long pseudo-random spreading code to a RAKE despreader. Theoutput of the RAKE despreader has a probability distribution function,(PDF). Referring to FIG. 1A, curve 1 shows a typical PDF background fornoise which is calculated using a long pseudo-random spreading codewhere there is no CW interference. Curve 3 shows a typical PDF in thepresence of a valid detected signal. However, when CW interference ispresent during the transmission of short codes, the background noise PDFis a curve like 2, which is shifted away from curve 1 and which appearssimilar to the PDF for a valid detected signal, curve 3. The noiseestimate becomes skewed because the short code, which is not completelyrandom is applied to the RAKE and it begins to correlate with therepetitive CW interference. Accordingly, as curve 2 shifts furthertoward curve 3 due to the presence of CW interference, the SPRTdetection method will falsely detect invalid noise as a valid signal.

[0009] Referring to FIG. 1B, there is shown a block diagram of a priorart short code detector system 10. The short code detector system 10 istypically located in a base station for detecting short codes receivedfrom a subscriber unit. A signal containing short codes, continuous waveinterference and other forms of background noise is applied to the shortcode detector system 10 by way of the detector input line 12, and isreceived by a detector input block 14. The detector input block 14includes a RAKE demodulator having M different phases. The RAKEdemodulator operates on the input signal by combining it with the shortpilot code. The pilot code is a pseudorandom code which is generatedlocally by the base station and transmitted by subscribers initiating acall setup.

[0010] A first output signal of the detector input block 14 is appliedto a detection block 16 of the detector system 10. The detection block16 contains a SPRT detection method. The output signal of the detectionblock 16 appears on a decision line 20. The signal of the decision line20 represents a decision by the SPRT detection method of detection block16 whether a short code is present in the signal received by the inputblock 14.

[0011] A second output signal of the input block 14 is applied to anoise estimator, which is comprised of a separate RAKE demodulator (AUXRAKE) which uses a long pseudorandom code in combination with the inputsignal to perform a background noise estimation. The result of thebackground noise estimation performed in block 18 is a PDF which isapplied to the SPRT detection method of detection block 16.

[0012] Referring now to FIG. 2, there is shown prior art short codedetection method 40. The detection method 40 is used to detect thepresence of short codes transmitted in a wireless communication system.For example, the short code detection method 40 is suitable foroperation within the detection block 16 of the short code detectorsystem 10 to detect the presence of short codes in the input signal ofthe input line 12.

[0013] Execution of the short code detection method 40 begins at thestart terminal 42 and proceeds to step 44 where one of the M differentphases of the RAKE 14 is selected. The short code detection method 40proceeds to step 46 where a background noise estimate, performed by theAUX RAKE, (in the noise estimator 18 of FIG. 1B), is updated. The signalis applied by the noise estimator 18 to the detection block 16. At step50, a sample of the signal from the selected phase of the input line 12as received by the input block 14 is applied to the detection block 16for computation according to the short code detection method 40.

[0014] Referring now to FIG. 3A, there is shown a graphicalrepresentation 70 of the operation of the short code detection method40. An acceptance threshold 74 and a rejection threshold 76 are setforth within along with two likelihood ratios 80, 84. A likelihood ratiois a decision variable that is well known to those skilled in the art.It is useful when determining the presence of a signal in acommunication system. The likelihood ratios 80, 84 have starting valuesapproximately midway between the thresholds 74, 76. They are repeatedlyadjusted by the short code detection method 40 for comparison withthresholds 74, 76 in order to determine the presence of short codes.

[0015] Although, the starting values of the likelihood ratios 80, 84 areapproximately midway between the thresholds 74, 76, adjustments are madeto the likelihood ratios 80, 84 which can be positive or negative asdetermined by the calculations of the detection method 40. As thelikelihood ratio of a phase increases and moves in the direction of theacceptance threshold 74, there is an increasing level of confidence thata short code is present. When a likelihood ratio crosses the acceptancethreshold 74 the level of confidence is sufficient to determine that ashort code is present in the phase. As the likelihood ratio decreasesand moves in the direction of the rejection threshold 76, there is anincreasing level of confidence that a short code is not present in thephase. When a likelihood ratio crosses the rejection threshold 76, thelevel of confidence is sufficient to determine that no short code ispresent.

[0016] Returning to FIG. 2 the likelihood ratio of the current phase isupdated at step 54. It will be understood by those skilled in the artthat such a likelihood ratio is calculated for each of the M differentphases of the RAKE. The likelihood ratio of the current phase iscalculated in view of the background estimate of step 46 and the inputsample taken at step 50.

[0017] At step 56, a determination is made whether the likelihood ratiosof all M phases are below the rejection threshold 76. If even one of thelikelihood ratios is above the rejection threshold 76 it is possiblethat a short code is present in the received transmission. In this case,execution of short code detection method 40 proceeds to step 58. At step58, a determination is made whether any of the likelihood ratioscalculated by the detection method 40 is above the acceptance threshold74. If any likelihood ratio is above acceptance threshold 74, asdetermined by step 58, a determination is made that a short code ispresent step 60.

[0018] If the detection method 40 operates within the detection block 16of the short code detector system 10 this determination can be indicatedby means of the decision line 20.

[0019] If all of the likelihood ratios are below the rejection threshold76 as determined by step 56, it is possible to be confident that noshort code is present in any of the M phases of the received signal.Accordingly, the detection method 40 proceeds to step 52 where thelikelihood ratios of all M phases are cleared. The phase of the localspreading code, the pilot code, is advanced in step 48 for use with theRAKE and the next RAKE phase is selected in step 44

[0020] If a likelihood ratio is above the rejection threshold 76 but nolikelihood ratio is above the acceptance threshold 74, as determined bystep 58, the detection method 40 proceeds by way of path 59 whereby anew sample of the signal phase is obtained, (step 50). The repeatedbranching of the detection method 40 by way of path 59 to obtain andprocess new samples in this manner causes the adjustment of the variouslikelihood ratios either toward or away from thresholds 74, 76. Theshort code detection method 40 repeatedly proceeds by way of path 59until either: 1) one of the likelihood ratios crosses above theacceptance threshold 74; or 2) all of the likelihood ratios cross belowthe rejection threshold 76. Only when one of these two events occurs isthere a sufficient confidence level to determine whether or not a shortcode is present. The number of samples required for one of these twoevents to occur is a measure of the efficiency of the short codedetection method 40.

[0021] Repeated branching by way of path 59 can provide either anincreasing likelihood or a decreasing likelihood that a short code ispresent. For example, in the case of the first likelihood ratio 80 shownin FIG. 3A, the repeated branching by way of path 59 causes adjustmentof likelihood ratio 80 generally in the direction of the rejectionthreshold 76. When continued performance of the operations of thedetection method 40 causes the likelihood ratio 80 to cross therejection threshold 76, there is a high enough confidence level todetermine that no short code is present within the current phase.Repeated branching by way of path 59 can also provide an increasinglikelihood that a short code is present. For example, in the case of thesecond likelihood ratio 84 shown in FIG. 3A, successive samples causeadjustment of the likelihood ratio 84 generally in the direction of theacceptance threshold 74. When continued branching by way of path 59causes the likelihood ratio 84 to cross the acceptance threshold 74,there is a high enough confidence level to determine that a short codeis present within the current phase.

[0022]FIG. 7 is a plot of the average number of samples required whenemploying the detection method 40 to acquire a short code in thepresence of CW interference. The plot demonstrates that the number ofsamples required to acquire a short code increases dramatically when theamplitude of CW interference is greater than 0.2 times the magnitude ofthe background noise. The drop in the number of samples shown for CWinterference greater than 0.6 times the magnitude of the backgroundnoise does not indicate improved short code detection performance, butrather, it reflects the fact that false detections begin occurring atthis point.

[0023] As shown in FIG. 7, low levels of CW background interferenceincrease short code acquisition time when using a conventional SPRTmethod, such as detection method 40. Additionally, higher levels of CWinterference cause false detections of short codes, which also result inan unacceptably long acquisition time to detect a valid short code. Theapplicant has recognized a need for a short code detection method thatcan reliably and quickly detect the presence of short codes in a CDMAtransmission that contains CW background noise.

SUMMARY OF THE INVENTION

[0024] A method is disclosed for receiving a transmitted signal in acommunication system employing CDMA techniques wherein the transmittedsignal includes a plurality of short codes, each of which is transmittedrepetitively over a fixed period of time. The method is particularlyuseful in rejecting CW interference which may be received with thetransmitted signal. The method includes using a SPRT for detecting thepresence of the short code in a plurality of phases of the receivedsignal by calculating a likelihood ratio for each phase. For each signalphase examined, the likelihood ratio is updated until its value eitherreaches a threshold that is consistent with the presence of a detectedshort code or reaches a threshold that is consistent with the absence ofa short code. A likelihood ratio is a comparison of the signal'sProbability Distribution Function (PDF) with a background noise PDF .The PDFs are calculated by passing the signal through a RAKE despreader.The background noise PDF is calculated by combining in the RAKE thecurrent short pilot code with the input signal. A new background noisePDF is calculated when the pilot code changes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1A is the probability distribution functions for receivedsignals and for background noise;

[0026]FIG. 1B is a prior art short code detector system;

[0027]FIG. 2 is a flow chart of a prior art short code detection methodsuitable for use in short code detection using the short code detectorsystem of FIG. 1B;

[0028]FIG. 3A is the likelihood ratios and decision thresholds suitablefor use in a short code detection method;

[0029]FIG. 3B is a block diagram of short codes;

[0030]FIG. 4A is the preferred embodiment of the present invention;

[0031]FIG. 4B is a flow chart of the short code detection method of thepresent invention;

[0032]FIG. 5 is a graph of the probability of false alarm performance ofthe prior art short code detection method of FIG. 1;

[0033]FIG. 6 is a graph of the probability of false alarm performance ofthe short code detection method of FIG. 4;

[0034]FIG. 7 is a graph of the average sample number performance of theprior art short code detection method of FIG. 1; and

[0035]FIG. 8 is a graph of the average sample number performance of theshort code detection method of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be described with reference to thedrawing figures wherein like numerals represent like elementsthroughout.

[0037] Referring to FIG. 4A, there is shown a block diagramrepresentation of the short code detector system 400 of the presentinvention. A signal containing short codes, continuous wave interferenceand other forms of background noise is applied to the short codedetector system 400 by way of a detector input line 412 and is receivedby a detector input block 414. The detector input block 414 includes aRAKE demodulator having M different phases.

[0038] A first output signal of the detector input block 414 is appliedto a detection block 416 of the detector system 400. The detection block416 contains a SPRT detection method. The output signal of the detectionblock 416 appears on a decision line 420. The signal of the decisionline 420 represents a decision by the SPRT detection method of thedetection block 416 whether a short code is present in the signalreceived by the input block 414. A second output signal of the detectorinput block 414 is applied to a noise estimator 418, which includes aseparate RAKE demodulator (AUX RAKE) which uses the same short codebeing transmitted by the subscriber. As will be explained in detailhereinafter, the body and noise estimate is used by the SPRT detectionmethod in the detection block 416 to more accurately detect the presenceof a valid signal code.

[0039] Referring to FIG. 4B, there is shown a short code detectionmethod 100 in accordance with the present invention. The short codedetection method 100 can be used in a short code detection system 400shown in FIG. 4A to detect the presence of short codes 88 a-c within thevarious phases of a received signal. Execution of the short codedetection method 100 begins at step 102 and proceeds to step 104 where abackground noise estimate is performed. The background noise estimate iscalculated by combining in the RAKE 414 the input signal with the shortcode; which is the same short code being used by subscriber to initiatecall setup to the base station.

[0040] Referring to FIG. 3B, there is shown a block diagram 86 of theshort codes 88 a-c which are used by the subscriber for transmission tothe base station. For example, a first short code 88 a is used for a 3millisecond duration as the input to the pilot RAKE. If the pilot signalhas not been detected by the base station within that 3 millisecond timeperiod, the short code 88 a is updated to a new short code 88 b. Anupdate period 92 b is necessary to update the code. Short codes areupdated every 3 millisecond to avoid any unexpected unfavorable crosscorrelation effects.

[0041] As will be explained in detail hereinafter, a new backgroundnoise estimate is calculated each time the short code 88 a-c used forthe detection of the pilot code changes. The use of periodically updatedshort codes by the present invention to estimate background noiseproduces a PDF that more closely resembles the actual background noise,even in the presence of continuous wave interference. Accordingly, asshown in FIG. 1A, curve 2 which represents the background noise in thepresence of CW interference is more readily distinguished from curve 3which represents a valid signal.

[0042] Referring to step 108, a plurality of phases M of the RAKE 14 isselected and a signal sample for each phase is obtained at step 116. Theinput (received) signal is despread using M different phases of theshort pilot code at the RAKE. In the present invention, the preferrednumber of phases M of the RAKE 14 is eight. However, it should beunderstood that any number may be selected The likelihood ratio for eachof the M phases is calculated at step 128, according to the backgroundnoise estimation of step 104 and the new samples of step 116. Since thepreferred embodiment of the present invention utilizes eight phases ofthe RAKE 14, calculations are performed in parallel for each phase.Accordingly, eight separate likelihood ratios are calculated andmaintained. A determination is made at step 138 whether the likelihoodratios of all M phases are below the rejection threshold 76. If thedetermination 138 is negative, a short code may be present in at leastone of the M phases. In this case, a further determination 144 is madewhether any of the likelihood ratios is above the acceptance threshold74. If the determination 144 is affirmative, a short code is present andexecution of short code detection method 100 proceeds to step 152 whichindicates that the pilot signal has been acquired.

[0043] If all of the likelihood thresholds are below the rejectionthreshold 76, as determined at step 138, there is a high enoughconfidence level to determine that no short codes are present in any ofthe current M phases. Under these circumstances, the detection method100 proceeds by way of branch 140 to step 134. At step 134 adetermination is made whether a three millisecond time period hasexpired.

[0044] The three millisecond time period at decision step 134 issynchronized with changes in the short codes used by the subscriber unitto acquire the pilot signal. The use of the three millisecond timeperiod in the present specification is by way of example only. Those ofskill in the art should realize that the time period used to update theshort codes for acquiring the pilot signal is the same time period thatwill be used in accordance with the present inventive method to updatethe background noise. The specific time period is not central to thepresent invention.

[0045] If the three millisecond timer has not expired, as determined atdecision step 134, detection continues using the same backgroundestimation. Under these circumstances execution of the short codedetection method 100 proceeds directly to step 120 where all of thelikelihood ratios for the current M phases are cleared. The code phaseis then advanced at step 112 and M new phases are processed, therebyrepeating step 108 and the short code detection method 100.

[0046] If the three millisecond time period has expired, as determinedby step 134, the timer is reset and a fast update of the backgroundnoise estimate is performed as shown at step 132. The background noiseestimate is performed in the manner previously described for step 104,using the new short code. The expiration of the 3 millisecond timeperiod coincides with the use of a new short code.

[0047] Referring back to FIG. 3B, since each short code 88 a-c has arespective update period 92 a-c at the beginning of the use of a newshort code 88 a-c, the background noise estimation update set forth atstep 132 is performed during the respective update period 92 a-c forthat short code. The sample of step 132 should be obtained very quicklyafter the time period expires. In the preferred embodiment of theinvention the sample is obtained within a few symbol periods of the useof a new short code 88 a-c.

[0048] This inventive method for updating the background noise resultsin performing the operations of short code detection method 100 upon aset of samples having a noise estimation using the same short code timeslot as the sample. The use of a noise estimation from the same timeslot as the sample improves the accuracy of short code detection method100. The background noise estimate is used to update a background noisePDF in step 124. At step 120 all likelihood ratios are cleared. Thelocal code phase is advanced at step 112 and a new phase and a newsample are processed, thereby repeating step 108 and beginning the shortcode detection method 100 again.

[0049] Referring back to FIG. 4B if a likelihood ratio is aboverejection threshold 76 but no likelihood ratios are above acceptancethreshold 74, as determined by step 144, execution of the short codedetection method 100 proceeds by way of branch 150 to step 148. At step148 a determination, is made whether the three millisecond time periodhas expired. If the three millisecond time period has not expired, thedetection method 100 continues to operate with the current backgroundnoise estimate, and a new sample for each of the M phases is taken atstep 116. If the three millisecond time period has expired, thisindicates that a new short code is being used. Accordingly, the timer isreset and a fast update of the background noise is performed in block146, the background noise estimate is adjusted in step 142, and a newsample for each phase is taken at step 116.

[0050] As described above, the three millisecond time period is testedduring every pass through the detection method 100, whether execution ofdetection method 100 passes by way of branch 140 where all currentlikelihood ratios have crossed the dismissal threshold, or whenexecution passes by way of branch 150 where no current likelihood ratioshave passed the acceptance threshold.

[0051] Referring to the graph 180 of FIG. 5, the graph 180 sets forththe probability of a false acquisition by prior art short code detectionmethod 40 for a plurality of values of CW magnitudes. The probability ofa false acquisition by the prior art short code detection method 40begins rising sharply when the CW interference is 0.5 times thenormalized value of the background noise and reaches one hundred percentwhen CW is at 0.8 times the value of the background noise.

[0052] However, referring to FIG. 6, a second graph 200 sets forth theprobability of a false acquisition by the present inventive short codedetection method 100 for a plurality of continuous wave magnitudes. Asshown, the probability of a false acquisition by the short codedetection method 100, is substantially zero even where CW interferenceis a large as 4 times the value of the background noise. Thus, thepresent invention provides a substantial improvement in falseacquisition performance over the prior art short code detection method40.

[0053] Referring now to FIGS. 7 and 8, two graphs are shown 220, 240which set forth the average sample number required by the short codedetection methods 40, 100 to determine whether a short code is present.It will be understood by those skilled in the art that the smaller thenumber of samples required to make this determination, the better themethod performs. As continuous wave interference magnitude increases,the prior art short code detection method 40 requires substantially moresamples in order to detect a short code. As shown in FIG. 7 the averagesample number can increase by an order of magnitude as the magnitude ofthe CW interference is increased. The drop in the number of samplesshown in graph 220 for CW interference greater than 0.6 times themagnitude of the background noise does not indicate improved short codedetection performance, but rather it reflects the fact that falsedetections begin occurring at this point.

[0054] In contrast, as shown in FIG. 8, the average sample numberrequired by the present inventive detection method 100 remainssubstantially constant over a wide range of continuous wave magnitudes.Furthermore, the required number of samples for the detection method 100remain substantially lower for CW magnitudes that are much higher thanthose causing the sharp rise in sample numbers for the prior artdetection method 40. False indications of short codes are virtuallyeliminated by the present invention.

[0055] The previous description of the preferred embodiments is providedin order to enable those skilled in the art to make and use the presentinvention. The various modifications to the embodiments shown will bereadily apparent to those skilled in the art, and the generic principlesdefined herein can be applied to other embodiments without providing aninventive contribution. Thus, the present invention is not intended tobe limited to the embodiments shown but is to be accorded the widestscope consistent with the principles and features disclosed.

What is claimed is:
 1. A system for receiving a transmitted signal whichincludes at least one short code which is periodically updated; thesystem comprising: a despreader for receiving and despreading saidtransmitted signal to output a despread signal; a background noiseestimator for obtaining a background noise estimation using said atleast one periodically updated short code; and a decision unit, whichreceives said despread signal and said background noise estimate,calculates a value representing the likelihood that a short code hasbeen detected and compares said value with a predetermined threshold;whereby the decision unit confirms the detection of said short code ifsaid value exceeds said predetermined threshold.
 2. The system of claim1, whereby the decision unit further compares said value with aplurality of predetermined thresholds, whereby at least one of saidpredetermined thresholds is an acceptance threshold and at least one ofsaid predetermined thresholds is a rejection threshold.
 3. The system ofclaim 2, whereby the transmitted signal has a plurality of signalphases, and whereby the decision unit compares a plurality of values,corresponding to the plurality of signal phases, with the plurality ofpredetermined thresholds.
 4. The system of claim 3, whereby the decisionunit advances the signal phase if one of said plurality of valuescrosses one of said plurality of predetermined thresholds.
 5. The systemof claim 3, whereby the decision unit advances the signal phase if oneof said plurality of values crosses said rejection threshold.
 6. Thesystem of claim 1, wherein the despreader includes a RAKE, and thedecision unit calculates said value in accordance with at least a sampleof the output of said RAKE.
 7. The system of claim 1, whereby thetransmitted signal comprises a plurality of time slots separated by aplurality of time slot boundaries and each time slot includes a timeslot update period, and the background noise estimator obtains saidbackground noise estimation during said update period.
 8. The system ofclaim 7, whereby the time slot update period occurs substantiallyimmediately after the time slot boundary.
 9. The system of claim 8,whereby said decision unit calculates said value during a selected timeslot in accordance with a background noise estimation obtained onlyduring said update period.
 10. A system for receiving a signaltransmitted by a communication unit wherein the transmitted signalincludes a plurality of short codes, and the communication unitrepetitively transmits at least one short code which is periodicallyupdated, the system comprising: a background noise estimator forobtaining a background noise estimation using the same periodicallyupdated short code; means for utilizing said background noise estimationto adjust a likelihood ratio in accordance with the transmitted signal;and a comparator for comparing said likelihood ratio with apredetermined threshold to determine whether said likelihood ratioexceeds said predetermined threshold.
 11. The system of claim 10,whereby the comparator further compares said ratio with a plurality ofpredetermined thresholds, whereby at least one of said predeterminedthresholds is an acceptance threshold and at least one of saidpredetermined thresholds is a rejection threshold.
 12. The system ofclaim 11, whereby the transmitted signal has a plurality of signalphases, and whereby the comparator compares a plurality of ratios,corresponding to the plurality of signal phases, with the plurality ofpredetermined thresholds.
 13. The system of claim 12, whereby thecomparator advances the signal phase if one of said plurality of ratioscrosses one of said plurality of predetermined thresholds.
 14. Thesystem of claim 13, whereby the comparator advances the signal phase ifone of said plurality of likelihood ratios crosses said rejectionthreshold.
 15. The system of claim 10, further comprising a RAKE,wherein the comparator utilizes the output of said RAKE and calculatessaid ratio in accordance with at least a sample of said output of saidRAKE.
 16. The system of claim 10, whereby the transmitted signalcomprises a plurality of time slots separated by a plurality of timeslot boundaries and each time slot includes a time slot update period,and the background noise estimator obtains said background noiseestimation during said update period.
 17. The system of claim 16,whereby the time slot update period occurs substantially immediatelyafter the time slot boundary.
 18. The system of claim 17, whereby saidcomparator calculates said ratio during a selected time slot inaccordance with a background noise estimation obtained only during saidupdate period.